btrfs: reduce kmap_atomic time for checksumming (443c8e2a) · Commits · 戴 / test

Since commit ("Btrfs: Compute and look up csums based on sectorsized blocks") we do a kmap_atomic() on the contents of a bvec. The code before had the kmap region just around the checksumming too. kmap_atomic() in turn does a preempt_disable() and pagefault_disable(), so we shouldn't map the data for too long. Reduce the time the bvec's page is mapped to when we actually need it. Performance wise it doesn't seem to make a huge difference with a 2 vcpu VM on a /dev/zram device: vanilla patched delta write 17.4MiB/s 17.8MiB/s +0.4MiB/s (+2%) read 40.6MiB/s 41.5MiB/s +0.9MiB/s (+2%) The following fio job profile was used in the comparision: [global] ioengine=libaio direct=1 sync=1 norandommap time_based runtime=10m size=100m group_reporting numjobs=2 [test] filename=/mnt/test/fio rw=randrw rwmixread=70 Signed-off-by:

Johannes Thumshirn <jthumshirn@suse.de> Reviewed-by:

David Sterba <dsterba@suse.com> Signed-off-by:

David Sterba <dsterba@suse.com>

fs/btrfs/file-item.c

+2 −6

Original line number	Diff line number	Diff line
		@@ -458,8 +458,6 @@ blk_status_t btrfs_csum_one_bio(struct inode inode, struct bio bio,
		BUG_ON(!ordered); /* Logic error */
		}

		data = kmap_atomic(bvec.bv_page);

		nr_sectors = BTRFS_BYTES_TO_BLKS(fs_info,
		bvec.bv_len + fs_info->sectorsize
		- 1);
		@@ -469,7 +467,6 @@ blk_status_t btrfs_csum_one_bio(struct inode inode, struct bio bio,
		offset < ordered->file_offset) {
		unsigned long bytes_left;

		kunmap_atomic(data);
		sums->len = this_sum_bytes;
		this_sum_bytes = 0;
		btrfs_add_ordered_sum(inode, ordered, sums);
		@@ -489,16 +486,16 @@ blk_status_t btrfs_csum_one_bio(struct inode inode, struct bio bio,
		sums->bytenr = ((u64)bio->bi_iter.bi_sector << 9)
		+ total_bytes;
		index = 0;

		data = kmap_atomic(bvec.bv_page);
		}

		sums->sums[index] = ~(u32)0;
		data = kmap_atomic(bvec.bv_page);
		sums->sums[index]
		= btrfs_csum_data(data + bvec.bv_offset
		+ (i * fs_info->sectorsize),
		sums->sums[index],
		fs_info->sectorsize);
		kunmap_atomic(data);
		btrfs_csum_final(sums->sums[index],
		(char *)(sums->sums + index));
		index++;
		@@ -507,7 +504,6 @@ blk_status_t btrfs_csum_one_bio(struct inode inode, struct bio bio,
		total_bytes += fs_info->sectorsize;
		}

		kunmap_atomic(data);
		}
		this_sum_bytes = 0;
		btrfs_add_ordered_sum(inode, ordered, sums);

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